My tank was more or less identical and I could happily run the drop checker so yellow it never went blue over night. I think I see your problem. You're not adding anywhere near enough ferts. I'd take a look at Georges iwagumi, I seem to remember he was adding 5ml or so daily. As ceg says a drop checker can only show you the path it's not a matter of right levels or wrong levels the plants will tell you what they need. Ferts wise I don't know the exact contents of TPN+ but it really does sound like you're massively under dosing for your tank. I'd be nervous with ferts with so many beautiful shrimp though. I know it's safe but they're touchy sods.